It is often desirable to bring a laser beam to a point of focus in order to raise the power density. Certain cutting or spot welding operations might be cited as examples. Focusing can be done with a conventional positive lens when power levels are low. At higher power levels, liquid cooled spherical or parabolic metal mirrors are often employed for focusing. When the focused laser beam is performing an operation such as cutting or spot welding, the work piece must be rather precisely located at the focal point. The beam expands on either side of this point and the power density falls accordingly. The size of the spot at the focal point and the working distance on either side of the focal point, in essence the tolerable depth of the field, are controlled by conventional rules of optics as well as the characteristics of the particular laser.
The limitation which requires the work piece to be at the focal point causes many problems. These may be complex geometric difficulties as are encountered when cutting or welding a three dimensional object. There may also be serious technical limitations as when attempting to make a deep cut in some material. One example of this is in cutting wood in order to reduce conventional sawing losses.
A solution to the depth of field problem would be to recollimate a focused beam by using a negative lens at or near the focal point. To date, the technology to do this has not been available except for very low powered lasers. Conventional lenses are simply pierced by a high power density beam. This is a special case of a more general problem associated with passing high power density laser beams through transparent solid materials. It applies not only to lenses, but to laser windows as well.
Since no optical material is 100% transmissive, it will absorb and convert to heat some of the energy of the beam passing through it. As long as this heat can be removed, no damage will result. The problem arises when heat build-up is more rapid than dissipation. This problem has become more acute as the power of industrial lasers has risen with the development of improved technology.
Windows were one of the first areas where alternatives to conventional optical elements had to be found. A window is the opening where the beam leaves the laser device. It serves to keep the medium inside the laser separate from the outside environment and is required because the lasing medium is most usually a gas of different composition and pressure than that in the outside environment.
One solution to the problem of conventional window destruction by high power density laser beams has been the use of so-called aerodynamic windows. A gas curtain is passed at very high velocity, normally supersonic, across the window opening. Typical early examples are shown in McLafferty, U.S. Pat. No. 3,604,789; and in U.S. Pat. Nos. 3,617,928 and 3,654,569 to Hausmann.
In later developments, the gas curtain has assumed the form of a segment of a free vortex. The nozzle creating the supersonic gas curtain can be designed so that the gas pressure on the laser side approximates the pressure within the laser while the pressure on the outside is typically at normal air pressure. In this way there is little or no transfer of gas into or out of the laser. U.S. Pat. Nos. 3,873,939 to Guile et al.; 3,973,217 to Guile; and 3,973,218 and 4,138,777 to Kepler et al. are representative.
Many references speak of problems arising from distortion of the laser beam as it passes through the supersonic gas window. McLafferty, in U.S. Pat. No. 4,112,388, attempts to overcome this problem by using adjacent layers of two different gases in his supersonic window. In effect, he creates a refractive index gradient across the window to minimize boundary layer disturbances.
It should be noted that in those aerodynamic windows employing free vortex segments, the laser beam passes essentially radially through the vortex segment.
Two other aerodynamic window types are exemplary of different approaches at reducing distortion as the beam passes through the window. One is seen in U.S. Pat. No. 4,201,952 to Stewart et al. which describes a window for use with a large diameter annular laser beam. It too uses a curtain of two different but adjacent gases, but the configuration is that of a radially expanding annulus. Griffin, in U.S. Pat. No. 3,918,800, shows two axially impinging gas columns through which a coaxial laser beam passes.
Gaseous curtains have found other laser-related applications besides aerodynamic windows. Hausmann, in U.S. Pat. No. 4,178,078, describes a cylindrical aerodynamic containment means for achieving better control of the plasma in a flashlamp for exciting a pulsed laser. Mack et al., U.S. Pat. No. 4,074,208, show a vortical gas containment system to accomplish flashlamp plasma containment.
Problems associated with focusing and collimating systems have apparently been much more intractible than those relating to windows, if one can judge by the relative dearth of pertinent literature. Watt, in U.S. Pat. No. 3,817,604, shows a system using conventional optics for bringing a laser beam to a focus. Ashkin et al., in U.S. Pat. Nos. 3,403,348 and 3,638,139, and Patel in U.S. Pat. No. 3,435,363, teach applications of "thermal" focusing. The laser beam is transmitted through a volume of a gas varying radially in temperature and thus also varying somewhat in index of refraction. If the temperature at the periphery is cooler than that at the axis, the effect is that of a weak negative lens. If the opposite situation holds and the peripheral temperature is higher, the effect is that of a weak positive lens. Welch, in U.S. Pat. No. 4,090,572, takes advantage of the positive lens effect to counteract divergence of a laser beam as it is projected down a deep borehole.
Brief comment is made here about the so-called "self-focusing" effect in lasers so that it is not confused with the present invention. The refractive index of any transmissive material varies with the intensity of the beam travelling through it. Because of this intensity dependence there is a tendency for a high power density beam having a gaussian energy distribution to collapse into a single spot because it is travelling slower in the center than along the edges. While some investigators have sought to take advantage of this characteristic, most have seen it as a problem to be overcome. The complex nature of this problem is reviewed by Campillo and Shapiro, Laser Focus, June, 1974, pp. 62-65.
Several other patents might be mentioned as having peripheral relationship to the present invention. Houldcroft, in U.S. Pat. No. 3,569,660; VanDer Jagt, in U.S. Pat. No. 3,685,882; and Diemer et al., in U.S. Pat. No. 4,121,085; all show gas-assisted lenses or nozzles for use in laser welding or cutting operations. These contain a positive lens for bringing the laser beam to a point of focus and a means for sweeping gas across the lens to cool and protect it by removing any vaporized products from the vicinity.
It is clear from a study of the literature that no consideration has been given to the use of a gaseous medium as a focusing device by any means other than establishment of a thermal gradient. The gas thermal lens, at best, shows only a small change in refractive index from axis to margin. It also poses many mechanical limitations as to where and how it can be used. These two considerations, taken together, have resulted in only very limited application of the thermal lens principle. Perturbations and disturbances in beam integrity have been noted where pressure gradients were encountered. However, the entire thrust of the prior art has been to seek means to eliminate this problem rather than to consider ways in which it might be used to advantage.